You can buy 1m drill bits very cheap , handy to keep .
https://www.toolstation.com/sds-plu...MIiuXmmNyg5AIVTLDtCh2DIwgWEAQYAyABEgI10_D_BwE
That is quite cheap, thanks. I'll have a go with the 12mm I've got, if that goes ok I'll pick up one of those 16mm bits to wide the hole.
Any reason I can't aim for a mortar joint between slate blocks, rather through a block? Would certainly make life easier!